The field of the present invention is lightweight, highly rigid connecting rods for internal combustion engines and their manufacture.
In recent years, the output power of internal combustion engines has been increased, and connecting rods used in these engines have been designed to be high in strength and rigidity and light in weight as well as practical. Connecting rods are formed by forging methods, casting methods, powder metallurgy methods and the like. The strength and rigidity of the connecting rods have been improved by sintering, nitriding, heat treatment, or the like. At the same time, connecting rods are partly or entirely formed with minimum cross sections in order to reduce weight.
However, in the formation of the connecting rods according to the above methods, there are restrictions in gradients for drawing patterns, sintering density, critical minimum thickness, etc. Therefore, it is difficult to attain desired minimum cross sections and weight.
FIGS. 1 and 2 illustrate one example of a conventional connecting rod or reduced weight. This connecting rod 01 is formed by the investment casting process. The rod 01 is designed to be slender in its entirety. The shoulder portion of the large end portion of the rod 01 is attached with a flange 03A connected with a flange 03 of the rod portion 02. This arrangement is such that the large end portion of the rod 01 exhibits substantial rigidity. However, since the rod portion 02 has an H-shaped section in which a web 04 is in parallel relation with a plane including an axis L.sub.1 of the crankshaft and an axis L.sub.2 on the piston pin, the rod portion 02 is necessarily thick compared with a rod portion having an I-shaped section with the web perpendicular to the aforementioned plane. Further, the moment of inertia of area of the I-shaped section is the same as that of the rod portion 02. In addition, since a slender side portion of the flange 03 is subjected to large bending moments due to the inertia of rotation, stress concentrations are apt to occur on the side portion.